The second in a four-part serieson physiological extremesencountered by Olympic athletes.

The gold medal, we like to think, goes to whoever is willing to suffer a bit more than everyone else.

So it's odd that, when physiologists at the University of Wisconsin used spinal injections of a powerful painkiller to block lower-body pain in a group of cyclists, the cyclists actually got slower. They initially felt great and started out faster than normal, but then faded. Without the feedback of pain, they couldn't pace themselves properly.

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Some of the most memorable Olympic moments involve athletes triumphing over pain: Silken Laumann's improbable bronze in 1992, just 10 weeks after a horrific rowing accident; Slovenian cross-country skier Petra Majdic breaking five ribs and collapsing a lung just minutes before her qualifying heat in Vancouver 2010, then skiing through it – four times! – to nab an excruciating bronze medal.

But as the Wisconsin experiment illustrates, pain is a much more complex phenomenon than our finger-on-a-hot-stove intuition tells us, and athletes in particular have a tangled relationship with it.

"Pain is more than one thing," explains Dr. Jeffrey Mogil, the head of the Pain Genetics Lab at McGill University. It's a sensation, like vision or touch; it's an emotion, like anger or sadness; and it's also a "drive state" that compels action, like hunger.

For athletes, all of these effects mingle together in different ways depending on the nature of the pain they're experiencing and the demands of their particular sport – the sudden shock of a stiff body-check versus the relentless burn of sustained effort.

For endurance athletes, pain at times seems like the central reality of their existence. In skill sports such as figure skating or snowboarding, on the other hand, pain is an undesirable and often avoidable, side effect, says Dr. Wolfgang Freund, a researcher at University Hospitals Ulm in Germany who studies pain in athletes. "Maradona at least had the illusion that a brilliant soccer player didn't need to suffer," he said.

Last year, Freund published a study on the pain tolerance of ultra-endurance runners competing in the TransEurope Footrace, an epic pain fest in which participants cover 4,487 kilometres over 64 days with no rest days. He asked 11 of the competitors to dunk their hands in ice water for three minutes; by the end, they rated the pain as about 6 out of 10 on average.

In contrast, the non-athlete control group gave up after an average of just 96 seconds when their pain maxed out at 10; only three of them even completed the test.

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The results fit with a growing body of evidence showing that athletes have greater pain tolerance than the general population. Interestingly, they seem to have roughly the same pain threshold: If you gradually turn up a stimulus such as heat or electric shocks, they'll start feeling pain at the same time as everyone else. They're just willing to endure it at higher levels, and for longer.

Some individual variation in pain sensitivity is genetic – Mogil pegs the fraction, very loosely, at a bit less than 50 per cent. He and his colleagues at McGill have identified 10 genes associated with pain response, and researchers elsewhere have identified another 50 or so, but there are still thousands more awaiting discovery. The rest of the variation is environmental, driven by factors such as age, diet and, most importantly, prior experience with pain. In general, the more pain you experience, the more sensitive to pain you become.

There are, however, some intriguing hints that the long hours of training endured by Olympic athletes produce the opposite effect. For example, a British Medical Journal study in 1981 found that elite swimmers displayed increasing tolerance to pain inflicted by cutting off circulation to their forearms as their training progressed toward a competitive peak. Their tolerance then declined again when they took a break from training.

To Dr. Alexis Mauger, a researcher at the University of Kent in Britain who is studying the relationship between pain and the limits of athletic performance, this suggests pain tolerance can indeed be trained. In part, he says, it's about: "Learning to break through a conservative pain barrier so that you can operate closer to a true physiological limit."

In other words, your brain tells you to stop before your body really has to. Mauger has led a series of studies in which cyclists taking acetaminophen (i.e., Tylenol) are able to cycle farther or faster than those given a placebo. The difference in speed is most pronounced late in the trials, when the cyclists are in the most pain.

Tellingly, the cyclists experience roughly the same levels of steadily increasing pain whether they get acetaminophen or placebo – it's only their pace that varies. This suggests that the pace you can sustain isn't set by some physiological limit, such as lactic acid in your muscles; instead, you're willing to endure a certain level of pain such that you reach the limit of tolerability right when you finish. And training, it seems, can raise this level.

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In some cases, athletes in the heat of competition become seemingly immune to pain – think Bobby Baun's 1964 overtime winner in the Stanley Cup finals on a broken leg, or Majdic's magical skiing medal. That's likely the result of "stress-induced analgesia," a phenomenon first observed in wounded soldiers during the U.S. Civil War.

In evolutionary terms, Mogil explains, pain serves a valuable function by telling you to stop and allow an injury to heal. "But if you're a deer being chased by a wolf and you trip and break a leg, you need to forget about that pain until later," he said.

For the most part, though, athletes feel pain like everyone else – and as the Wisconsin study illustrates, pain also offers crucial feedback about how your body is responding to the challenge. The nerve signals from aching legs not only trigger discomfort, but also warn your body to adjust breathing and circulation to deliver more oxygen to tired muscles.

The end result: You can't pace yourself, or win a race, without pain. So the gold medalist isn't necessarily the athlete who suffers the most, after all. He or she is the one who uses the pain best.